Photo electrochemical cell for water splitting

ABSTRACT

A GaON/ZnO photoelectrode involving a nanoarchitectured photocatalytic material deposited onto a surface of a conducting substrate, and the nanoarchitectured photocatalytic material containing gallium oxynitride nanoparticles interspersed in zinc oxide nanoparticles, as well as methods of preparing the GaON/ZnO photoelectrode. A method of using the GaON/ZnO photoelectrode for solar water electrolysis is also provided.

STATEMENT OF FUNDING ACKNOWLEDGEMENT

This project was prepared with support from the National Plan forScience, Technology, and Innovation (MAARIFAH) of King Abdulaziz Cityfor Science and Technology through the Science and Technology Unit atKing Fahd University of Petroleum and Minerals (KFUPM), the Kingdom ofSaudi Arabia, under award No. 13-NAN1600-04.

STATEMENT REGARDING PRIOR DISCLOSURE BY THE INVENTORS

Aspects of this technology are described in an article “Single-stepstrategy for the fabrication of GaON/ZnO nanoarchitectured photoanodetheir experimental and computational photoelectrochemical watersplitting” published in Nano Energy, 2018, 44, 23-33, on Dec. 2, 2017,which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION Technical Field

The present disclosure relates to a GaON/ZnO photoelectrode and methodsof its preparation. The present disclosure further relates to a methodof using the GaON/ZnO photoelectrode as part of a photoelectrochemicalcell for water splitting.

Description of the Related Art

The “background” description provided herein is for the purpose ofgenerally presenting the context of the disclosure. Work of thepresently named inventors, to the extent it is described in thisbackground section, as well as aspects of the description which may nototherwise qualify as prior art at the time of filing, are neitherexpressly or impliedly admitted as prior art against the presentinvention.

As a renewable energy substitute for fossil fuels, solar radiation mayalleviate the worldwide energy constraint without posing anenvironmental threat. Photoelectrochemical (PEC) water splitting isconsidered as a solution to generate hydrogen gas using renewable energysources including water and sunlight, which are both abundant. Inprinciple, semiconductor photoelectrodes capture sunlight photons togenerate photoexcitons, i.e. electrons (e⁻) and holes (h⁺) in theirconduction (CB) and valence bands (VB), respectively. The photoexcitonshaving an adequate energy would initiate redox reactions and split waterinto H₂ and O₂ gases. Fujishima and Honda accomplished overall watersplitting using a TiO₂ photocatalyst under ultraviolet radiations [A.Fujishima, K. Honda, Electrochemical photolysis of water at asemiconductor electrode, Nature 238 (1972) 37-38]. There has beenintense research activity on metals, semiconductors, and organicconjugated systems in trying to shift the activation wavelength of thePEC process to the visible region [R. Xu, H. Li, W. Zhang, Z. Yang, G.Liu, Z. Xu, H. Shao, G. Qiao, The fabrication of In₂O₃/In₂S₃/Agnanocubes for efficient photoelectrochemical water splitting, Phys.Chem. Chem. Phys. 18 (2016) 2710-2717; V. Avasare, Z. Zhang, D. Avasare,I. Khan, A. Qurashi, Roomtemperature synthesis of TiO₂ nanospheres andtheir solar driven photoelectrochemical hydrogen production, Int. J.Energy Res. 39 (2015) 1714-1719; and N. Iqbal, I. Khan, Z.H. Yamani, A.Qurashi, Sonochemical assisted solvothermal synthesis of galliumoxynitride nanosheets and their solar-driven photoelectrochemicalwater-splitting applications, Sci. Rep. 6 (2016) 32319, eachincorporated herein by reference in their entirety]. However, thefabrication of an efficient and stable PEC water splitting systemremains a fundamental issue relating to PEC technique. Additionally,certain factors such as limited light absorption, insufficientgeneration of photoexcitons (e⁻ and h⁺) and their separation, andlimited water redox reactions limit the performance of PEC watersplitting catalyst, and hence decrease the overall efficiency of theenergy transfer process. In order to overcome these problems, manyapproaches such as tandem cell fabrication, hybrid/composite structureformations, nitrogen doping, as well as decorating the surface ofphotoelectrode with stable and efficient co-catalysts such as Ag and Audue to their enhanced water redox reaction kinetics were attempted [S.Esiner, G.W.P. van Pruissen, M.M. Wienk, R.A.J. Janssen, G.W.P. vanPruissen, M.M. Wienk ab, R.A.J. Janssen, G.W.P. van Pruissen, M.M.Wienk, R.A.J. Janssen, Optimized light-driven electrochemical watersplitting with tandem polymer solar cells, J. Mater. Chem. A 4 (2016)5107-5114; U. Veikko, X. Zhang, T. Peng, P. Cai, G. Cheng, The synthesisand characterization of dinuclear ruthenium sensitizers and theirapplications in photocatalytic hydrogen production, Spectrochim. ActaPart A Mol. Biomol. Spectrosc. 105 (2013) 539-544; C. Nguyen Van, T.H.Do, J.-W. Chen, W.-Y. Tzeng, K.-A. Tsai, H. Song, H.-J. Liu, Y.-C. Lin,Y.-C. Chen, C.-L. Wu, C.-W. Luo, W.-C. Chou, R. Huang, Y.-J. Hsu, Y.-H.Chu, WO₃ mesocrystal-assisted photoelectrochemical activity of BiVO₄,NPG Asia Mater. 9 (2017) e357; and T. Hisatomi, J. Kubota, K. Domen,Recent advances in semiconductors for photocatalytic andphotoelectrochemical water splitting, Chem. Soc. Rev. 43 (2014)7520-7535, each incorporated herein by reference in their entirety].Even though some approaches have enhanced the efficiency of PEC watersplitting, further research is necessary to make the production ofhydrogen gas more economical. In general, a solar junction cell would beconsidered efficient if the photoelectrode is robust to photocorrosionand capable of splitting water without the assistance of a co-catalyst.Thus, high-performance multi-component photoactive materials are needed.

With n-type semi-conductor characteristics including an indirect bandgap of 3.37 eV, ZnO is a cost-effective alternative to TiO₂. As revealedfrom a comparative study by Hernández et al., ZnO showed improvedphotocurrent densities compared to TiO₂ [S. Hernández, D. Hidalgo, A.Sacco, A. Chiodoni, A. Lamberti, V. Cauda, E. Tresso, G. Saracco,Comparison of photocatalytic and transport properties of TiO₂ and ZnOnanostructures for solar-driven water splitting, Phys. Chem. Chem. Phys.17 (2015) 7775-7786, incorporated herein by reference in its entirety].ZnO, which has a band edge straddling the water oxidation potentiallevel, is a good photocatalyst for oxygen evolution reactions (OER).However, due to high recombination rate of its excitons, the quantumyield of ZnO is still insufficient [Z.-Q. Liu, P.-Y. Kuang, R.-B. Wei,N. Li, Y.-B. Chen, Y.-Z. Su, BiOBr nanoplate-wrapped ZnO nanorod arraysfor high performance photoelectrocatalytic application, RSC Adv. 6(2016) 16122-16130, incorporated herein by reference in its entirety],which makes the implementation of ZnO in its pristine form challenging.Therefore, substantial efforts have been devoted to improve thephoto-efficiency of ZnO through doping [H.-J. Choi, S.-J. Choi, S. Choo,I.-D. Kim, H. Lee, Hierarchical ZnO nanowires-loaded Sb-doped SnO₂—ZnOmicrograting pattern via direct imprinting-assisted hydrothermal growthand its selective detection of acetone molecules, Sci. Rep. 6 (2016)18731, incorporated herein by reference in its entirety], or by makingheterostructural composites [J. Lee, K. Yong, Combining the lotus leafeffect with artificial photosynthesis: regeneration of underwatersuperhydrophobicity of hierarchical ZnO/Si surfaces by solar watersplitting, NPG Asia Mater. 7 (2015) e201; and I. Khan, A.A.M. Ibrahim,M. Sohail, A. Qurashi, Sonochemical assisted synthesis of RGO/ZnOnanowire arrays for photoelectrochemical water splitting, Ultrason.Sonochem. 37 (2017) 669-675, each incorporated herein by reference intheir entirety] with additional materials having suitable bandgaps.

The photocatalytic properties of pristine semiconductors can beincreased by loading appropriate noble metal co-catalysts and/ortransition metal oxides. As an example, GaN:ZnO was tested for overallPEC water splitting, owing to its good visible light absorption andphotocorrosion resistance characteristics [K. Maeda, T. Takata, M. Hara,N. Saito, Y. Inoue, H. Kobayashi, K. Domen, GaN:ZnO solid solution as aphotocatalyst for visible-light-driven overall water splitting, J. Am.Chem. Soc. 127 (2005) 8286-8287, incorporated herein by reference in itsentirety]. Loading 5 wt% RuO₂ to GaN:ZnO further enhanced thephotocatalytic activity of the overall catalytic system. Suitable bandedge alignments with the thermodynamic OER and hydrogen evolutionreaction (HER) energy bands enable a better photocatalytic performanceon overall PEC water splitting. Continued effort in syntheticmodification has led to a PEC water splitting photocatalyst having along lifetime, which is active under visible solar light with the aid ofRh_(2-y)Cr_(y)O₃ co-catalysts [T. Ohno, L. Bai, T. Hisatomi, K. Maeda,K. Domen, Photocatalytic water splitting using modified GaN:ZnO solidsolution under visible light: long-time operation and regeneration ofactivity, J. Am. Chem. Soc. 134 (2012) 8254-8259, incorporated herein byreference in its entirety]. However, due to undesirable recombinationbetween (e⁻) and (h⁺), a high efficiency is not yet achieved. Thefabrication of ZnO:GaN material may be a useful approach to reduce thecharge recombination and suppress the electron diffusion through theZnO:GaN/electrolyte boundary. Furthermore, methods including nitridationand assembling these materials onto a conductive electrode such as ITO,FTO etc. would enable exterior electric field to improve the chargeseparation [S. Yu, B. Liu, Q. Wang, Y. Gao, Y. Shi, X. Feng, X. An, L.Liu, J. Zhang, Ionic liquid assisted chemical strategy to TiO₂ hollownanocube assemblies with surface-fluorination and nitridation and highenergy crystal facet exposure for enhanced photocatalysis, ACS Appl.Mater. Interfaces 6 (2014) 10283-10295, incorporated herein by referencein its entirety].

Gallium oxynitride (GaON) may be a suitable additive to ZnO. It isnitrogen- and oxygen-rich. With a proper amount of doped nitrogen, GaONshowed improved optical properties [N. Iqbal, 1. Khan, Z.H. Yamani, A.Qurashi, Sonochemical assisted solvothermal synthesis of galliumoxynitride nanosheets and their solar-driven photoelectrochemicalwater-splitting applications, Sci. Rep. 6 (2016) 32319; and C.-C. Hu, H.Teng, Gallium oxynitride photocatalysts synthesized from Ga(OH)₃ forwater splitting under visible light irradiation, J. Phys. Chem. C 114(2010) 20100-20106, each incorporated herein by reference in theirentirety]. Lately, Delaunay and Domen et al. fabricated a ZnO-ZnGaONphotoanode composite using a complicated and challenging chemical vapordeposition (CVD) method at a relatively high temperature, i.e. 600° C. Aphotocurrent efficiency of about 1.5 mA/cm² was achieved [M. Zhong, Y.Ma, P. Oleynikov, K. Domen, J.-J. Delaunay, A conductive ZnO-ZnGaONnanowire-array-on-a-film photoanode for stable and efficient sunlightwater splitting, Energy Environ. Sci. 7 (2014) 1693, incorporated hereinby reference in its entirety]. More recently, a simple solvothermalsynthesis and photoelectrochemical water splitting properties of GaONnanosheets were reported [N. Iqbal, I. Khan, Z.H. Yamani, A. Qurashi,Sonochemical assisted solvothermal synthesis of gallium oxynitridenanosheets and their solar-driven photoelectrochemical water-splittingapplications, Sci. Rep. 6 (2016) 32319, incorporated herein by referencein its entirety]. However, the photocurrent density of the GaONnanosheets was lower than pristine GaON.

In view of the forgoing, one objective of the present invention is toprovide a GaON/ZnO photoelectrode based on a photocatalytic materialhaving gallium oxynitride nanoparticles interspersed in zinc oxidenanoparticles, and a method for making thereof. The GaON/ZnOphotoelectrode may be used in a photoelectrochemical cell for producinghydrogen and oxygen gases.

BRIEF SUMMARY OF THE INVENTION

According to a first aspect, the present disclosure relates to aGaON/ZnO photoelectrode, including a metal oxide conducting substrate,and a nanoarchitectured photocatalytic material deposited onto a surfaceof the metal oxide conducting substrate, wherein the nanoarchitecturedphotocatalytic material comprises zinc oxide nanoparticles, and galliumoxynitride nanoparticles interspersed in the zinc oxide nanoparticles.

In one embodiment, the zinc oxide nanoparticles are in the form ofnanorods.

In one embodiment, the gallium oxynitride nanoparticles are in the formof nanosheets.

In one embodiment, the nanorods have an average diameter of 20-100 nmand an average length of 500-2,000 nm.

In one embodiment, the nanosheets have an average thickness of 5-50 nmand an average length of 200-1,500 nm.

In one embodiment, a weight ratio of the zinc oxide nanoparticles to thegallium oxynitride nanoparticles is in a range of 4:1 to 100:1.

In one embodiment, the metal oxide conducting substrate is fluorinedoped tin oxide substrate.

In one embodiment, the GaON/ZnO photoelectrode has an ultravioletvisible absorption with an absorption edge of 410-520 nm.

In one embodiment, the GaON/ZnO photoelectrode has a band gap energy of2.2-2.7 eV.

According to a second aspect, the present disclosure relates to aphotoelectrochemical cell containing the GaON/ZnO photoelectrode of thefirst aspect, a counter electrode, and an electrolyte solutioncomprising water and an inorganic salt in contact with both the GaON/ZnOphotoelectrode and the counter electrode.

In one embodiment, the electrolyte solution has an inorganic saltconcentration of 0.05-1 M.

In one embodiment, the GaON/ZnO photoelectrode has a photo-currentdensity in a range from 0.01-1.5 mA/cm² when the photoelectrochemicalcell is subjected to a potential of 0.1 to 1.5 V under visible lightirradiation.

In one embodiment, the photoelectrochemical cell further comprises areference electrode.

According to a third aspect, the present disclosure relates to a methodof splitting water into hydrogen gas and oxygen gas. The method involvesthe steps of subjecting the photoelectrochemical cell of the secondaspect to a potential of 0.5 to 2.0 V, and concurrently irradiating thephotoelectrochemical cell with visible light, thereby forming hydrogengas and oxygen gas.

According to a fourth aspect, the present disclosure relates to a methodof producing the GaON/ZnO photoelectrode of the first aspect. The methodinvolves the steps of mixing zinc oxide nanoparticles and galliumoxynitride nanoparticles in a solvent to form a mixture, sonicating themixture to form a dispersed mixture, depositing the dispersed mixtureonto a surface of a metal oxide conducting substrate to form a depositedsubstrate, and heating the deposited substrate at a temperature of70-150° C. for 0.5-4 hours.

In one embodiment, a weight ratio of the zinc oxide nanoparticles to thegallium oxynitride nanoparticles is in a range of 4:1 to 100:1.

In one embodiment, the solvent is water.

In one embodiment, the depositing is performed at a temperature of50-120° C.

In one embodiment, the depositing is performed at a pressure of 100-300KPa.

In one embodiment, the heating is followed by cooling to a temperatureranging from 5-40° C. thereby forming the GaON/ZnO photoelectrode.

The foregoing paragraphs have been provided by way of generalintroduction, and are not intended to limit the scope of the followingclaims. The described embodiments, together with further advantages,will be best understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the disclosure and many of the attendantadvantages thereof will be readily obtained as the same becomes betterunderstood by reference to the following detailed description whenconsidered in connection with the accompanying drawings, wherein:

FIG. 1A shows an optimized unit cell of GaON.

FIG. 1B shows slab geometries with O-rich surface orientation withoutZn₃O₃ cluster.

FIG. 1C shows slab geometries with O-rich surface orientation with Zn₃O₃cluster.

FIG. 2 is a schematic demonstration for the formation of GaON/ZnOnanoarchitecture.

FIG. 3 shows X-ray diffraction (XRD) patterns of ZnO nanorods, GaONnanosheets, and GaON/ZnO nanoarchitecture.

FIG. 4A is a field emission scanning electron microscopy (FE-SEM) imageof GaON nanosheets.

FIG. 4B is a magnified view of the sample in FIG. 4A.

FIG. 4C is a FE-SEM image of GaON/ZnO nanoarchitecture.

FIG. 4D is a magnified view of the sample in FIG. 4C.

FIG. 5A shows an energy-dispersive X-ray spectroscopy (EDX) analysis ofGaON/ZnO nanoarchitecture.

FIG. 5B shows a Zn elemental mapping of GaON/ZnO nanoarchitecture.

FIG. 5C shows an O elemental mapping of GaON/ZnO nanoarchitecture.

FIG. 5D shows an N elemental mapping of GaON/ZnO nanoarchitecture.

FIG. 5E shows a Ga elemental mapping of GaON/ZnO nanoarchitecture.

FIG. 6A shows an X-ray photoelectron spectroscopy (XPS) spectrum ofGaON/ZnO nanoarchitecture for nitrogen (1 s).

FIG. 6B shows an X-ray photoelectron spectroscopy (XPS) spectrum ofGaON/ZnO nanoarchitecture for oxygen (1 s).

FIG. 6C shows an X-ray photoelectron spectroscopy (XPS) spectrum ofGaON/ZnO nanoarchitecture for zinc (2p).

FIG. 6D shows an X-ray photoelectron spectroscopy (XPS) spectrum ofGaON/ZnO nanoarchitecture for gallium (3d).

FIG. 6E shows an X-ray photoelectron spectroscopy (XPS) spectrum ofGaON/ZnO nanoarchitecture for gallium (2p 3/2).

FIG. 7 is an overlay of Fourier-transform Infrared (FTIR) vibrationalspectra of ZnO nanorods, GaON nanosheets, and GaON/ZnO nanoarchitecture.

FIG. 8A is an overlay of diffuse reflectance ultraviolet-visible(DRS-UV-vis) absorption spectra of ZnO nanorods, GaON nanosheets, andGaON/ZnO nanoarchitecture.

FIG. 8B is an overlay of Kubelka-Munk (KM) bandgap energies of ZnOnanorods, GaON nanosheets, and GaON/ZnO nanoarchitecture.

FIG. 8C is an overlay of photoluminescence spectra of ZnO nanorods, andGaON/ZnO nanoarchitecture.

FIG. 9A is an overlay of linear sweep voltammograms (LSV) of ZnOnanorods, GaON nanosheets, and GaON/ZnO nanoarchitecture deposited overFTO substrate, in a standard three electrode system under dark (“Dark”)and simulated solar light (1 SUN) (“Light”).

FIG. 9B is an overlay of chronoamperometric measurements of ZnOnanorods, GaON nanosheets, and GaON/ZnO nanoarchitecture deposited overFTO substrate in which photocurrent densities were recorded at 1.23 Vvs. saturated calomel electrode (SCE) bias in a standard three electrodesystem under dark (“ Light OFF”) and simulated solar light (1 Sun)(“Light ON”).

FIG. 9C shows stability measurements of ZnO nanorods, GaON nanosheets,and GaON/ZnO nanoarchitecture deposited over FTO substrate by monitoringtheir current densities over a period of 1250 seconds.

FIG. 9D is an overlay of Nyquist plots of ZnO nanorods, GaON nanosheets,and GaON/ZnO nanoarchitecture deposited over FTO substrate.

FIG. 10 shows the photoelectrochemical (PEC) water splitting mechanismover GaON/ZnO photoelectrode.

FIG. 11A is a bar graph showing adsorption energies of water molecule onO-rich (111) surface of GaON and on GaON/ZnO hetero-structure.

FIG. 11B shows an optimized structure of water molecules (larger balls)on the surface of GaON.

FIG. 11C shows another optimized structure of water molecules (largerballs) on the surface of GaON.

FIG. 11D shows another optimized structure of water molecules (largerballs) on the surface of GaON.

FIG. 11E shows an optimized structure of water molecules (larger balls)on the surface of GaON/ZnO.

FIG. 11F shows another optimized structure of water molecules (largerballs) on the surface of GaON/ZnO.

FIG. 11G shows another optimized structure of water molecules (largerballs) on the surface of GaON/ZnO.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Embodiments of the present disclosure will now be described more fullyhereinafter with reference to the accompanying drawings, in which some,but not all embodiments of the disclosure are shown.

The present disclosure may be better understood with reference to thefollowing definitions. As used herein, the words “a” and “an” and thelike carry the meaning of “one or more.” Within the description of thisdisclosure, where a numerical limit or range is stated, the endpointsare included unless stated otherwise. Also, all values and subrangeswithin a numerical limit or range are specifically included as ifexplicitly written out.

The present disclosure further includes all isotopes of atoms occurringin the present compounds. Isotopes include those atoms having the sameatomic number but different mass numbers. By way of general example, andwithout limitation, isotopes of hydrogen include deuterium and tritium,isotopes of oxygen include ¹⁶O, ¹⁷O and ¹⁸O, isotopes of nitrogeninclude ¹⁴N, and ¹⁵N, stable isotopes of gallium include ⁶⁹Ga, and ⁷¹Ga,and stable isotopes of zinc include ⁶⁴Zn, ⁶⁶Zn, ⁶⁷Zn, ⁶⁸Zn, and ⁷⁰Zn.Isotopically labeled compounds of the disclosure can generally beprepared by conventional techniques known to those skilled in the art orby processes and methods analogous to those described herein, using anappropriate isotopically labeled reagent in place of the non-labeledreagent otherwise employed.

According to a first aspect, the present disclosure relates to aGaON/ZnO photoelectrode, including a metal oxide conducting substrate,and a nanoarchitectured photocatalytic material deposited onto a surfaceof the metal oxide conducting substrate.

A particle is defined as a small object that behaves as a whole unitwith respect to its transport and properties. An average diameter (e.g.,average particle diameter) of the particle, as used herein, and unlessotherwise specifically noted, refers to the average linear distancemeasured from one point on the particle through the center of theparticle to a point directly across from it. For polygonal shapes, theterm “diameter”, as used herein, and unless otherwise specified, refersto the greatest possible distance measured from a vertex of a polygonthrough the center of the face to the vertex on the opposite side. For acircle, an oval, an ellipse, and a multilobe, “diameter” refers to thegreatest possible distance measured from one point on the shape throughthe center of the shape to a point directly across from it.

Nanoparticles are particles between 1 and 100 nm in size. Theexceptionally high surface area to volume ratio of nanoparticles maycause the nanoparticles to exhibit significantly different or even novelproperties from those observed in individual atoms/molecules, fineparticles and/or bulk materials. Nanoparticles may be classifiedaccording to their dimensions. Three-dimensional nanoparticlespreferably have all dimensions of less than 100 nm, and generallyencompass isodimensional nanoparticles. Examples of three dimensionalnanoparticles include, but are not limited to nanoparticles,nanospheres, nanogranules and nanobeads. Two-dimensional nanoparticleshave two dimensions of less than 100 nm, generally including diameter.Examples of two-dimensional nanoparticles include, but are not limitedto, nanosheets, nanoplatelets, nanolaminas and nanoshells.One-dimensional nanoparticles have one dimension of less than 100 nm,generally thickness. Examples of one-dimensional nanoparticles include,but are not limited to, nanorods, nanotubes, nanofibers andnanowhiskers.

The nanoarchitectured photocatalytic material of the presently disclosedGaON/ZnO photoelectrode comprises zinc oxide in the form of particles ofthe same shape or different shapes, and of the same size or differentsizes. In one or more embodiments, the zinc oxide particles are in theform of nanoparticles. The zinc oxide nanoparticles may beone-dimensional, two-dimensional, three-dimensional nanoparticles ormixtures thereof. Preferably, the zinc oxide nanoparticles areone-dimensional nanoparticles. In an alternative embodiment, the zincoxide nanoparticles used in the present disclosure may have one or moredimensions greater than 100 nm. In preferred embodiments, the zinc oxidenanoparticles are in the form of nanorods (zinc oxide nanorods).Nanorods, also termed as nanowires, are a one-dimensional nanostructurewith a standard aspect ratio (length divided by width or diameter) ofabout 3-5. Depending on factors such as material identities andpreparation techniques, nanorods may have an aspect ratio that isgreater than 5.

The cross-section of the zinc oxide nanorods may be of any desiredshape, such as a circle, an oval, an ellipse, a multilobe, and apolygon. In a preferred embodiment, the cross-section of the zinc oxidenanorods is rectangular shaped. In another preferred embodiment, thecross-section of the zinc oxide nanorods is polygonal shaped. As definedherein, a diameter of a zinc oxide nanorod is measured at the pointalong the nanorod where the cross section is the widest. An averagediameter of the zinc oxide nanorods refers to the average of diametersof each nanorod present in the nanoarchitectured photocatalyticmaterial. In one or more embodiments, the zinc oxide nanorods have anaverage diameter of 20-200 nm, preferably 40-150 nm, more preferably60-100 nm. However, in certain embodiments, the average diameter of thezinc oxide nanorods is less than 20 nm, or greater than 200 nm. In oneor more embodiments, the zinc oxide nanorods have an average length of500-2,000 nm, preferably 800-1,600 nm, more preferably 1000-1,200 nm.However, in certain embodiments, the average length of the zinc oxidenanorods is less than 500 nm, or greater than 2,000 nm. In oneembodiment, the zinc oxide nanorods have an aspect ratio of 3-200,5-100, 10-50, or 20-40. The cross-section of the zinc oxide nanorods maybe constant over the length of the nanorods or may vary over the length.

In one embodiment, the zinc oxide nanorods are uniform throughout theentire length of the nanorods and are of a cylindrical shape. In anotherembodiment, the zinc oxide nanorods are conical-shaped or elongatedoval-shaped (cigar-shaped). In a preferred embodiment, the zinc oxidenanorods have rectangular or polygonal shaped cross-sections withdiameters that taper along the length of the nanorod to a rounded tip(see FIG. 4D).

The photocatalytic material of the presently disclosed GaON/ZnOphotoelectrode comprises gallium oxynitride in the form of particles ofthe same shape or different shapes, and of the same size or differentsizes. In one or more embodiments, the gallium oxynitride particles arein the form of nanoparticles. The gallium oxynitride nanoparticlespreferably are two-dimensional nanoparticles but may be one-dimensional,two-dimensional, three-dimensional or mixtures thereof. In analternative embodiment, the gallium oxynitride nanoparticles used in thepresent disclosure may have one or more dimensions greater than 100 nm.In preferred embodiments, the gallium oxynitride nanoparticles are inthe form of nanosheets (gallium oxynitride nanosheets). Nanosheets, alsoknown as ultrathin film, have a two-dimensional nanostructure with alarge surface area to thickness ratio.

The gallium oxynitride nanosheets may be of any desired shape, such as atriangle, a rectangle, a pentagon, a hexagon, or an irregular polygon.In a preferred embodiment, the gallium oxynitride nanosheets haveirregular or scalloped edges. In one or more embodiments, the galliumoxynitride nanosheets have an average thickness of 5-80 nm, preferably10-50 nm, more preferably 20-40 nm. However, in certain embodiments, theaverage thickness of the gallium oxynitride nanosheets is less than 5nm, or greater than 80 nm. In one or more embodiments, the galliumoxynitride nanosheets have an average length of 200-1,500 nm, preferably400-1,200 nm, more preferably 600-1,000 nm. In certain embodiments, theaverage length of the gallium oxynitride nanosheets is less than 200 nm,or greater than 1,500 nm. The gallium oxynitride nanosheets may beagglomerated or non-agglomerated (i.e., the gallium oxynitridenanosheets are well separated from one another and do not formclusters). In one embodiment, the gallium oxynitride nanosheets areagglomerated and the agglomerates have an average diameter in a range of1-50 µm, 2-25 µm, or 5-10 µm. In a preferred embodiment, the galliumoxynitride nanosheet agglomerate has a petal-like arrangement (see FIG.4B).

In one or more embodiments, the gallium oxynitride nanoparticles (e.g.nanosheets) are interspersed in the zinc oxide nanoparticles (e.g.nanorods). The gallium oxynitride nanoparticles may interact with thezinc oxide nanoparticles via van der Waals forces and/or electrostaticforces. In one or more embodiments, the gallium oxynitride nanoparticlesare randomly located in the zinc oxide nanoparticles, i.e. distancesbetween a gallium oxynitride nanoparticle and its neighboring galliumoxynitride nanoparticles are different. Alternatively, the galliumoxynitride nanoparticles are evenly arranged in the zinc oxidenanoparticles, i.e. a distance between a gallium oxynitride nanoparticleand all its neighbors is the same or substantially the same. Thedistance can be said to be substantially the same when the shortestdistance is at least 80%, at least 85%, at least 90%, or at least 95% ofthe average distance and the longest distance is not more than 120%, notmore than 110%, or not more than 105% of the average distance. Thedistance is measured from a center of a gallium oxynitride nanoparticleto a center of a neighboring gallium oxynitride nanoparticle and may bein a range of 1 nm to 1 µm, 10-800 nm, 50-600 nm, 100-400 nm, or 200-300nm. Energy-dispersive X-ray spectroscopy, X-ray microanalysis, elementalmapping, transmission electron microscopy, scanning electron microscopy(see FIGS. 4C-D in Example 3), and scanning transmission electronmicroscopy may be useful techniques for observing the arrangement ofnanoparticles.

In one or more embodiments, a weight ratio of the zinc oxidenanoparticles to the gallium oxynitride nanoparticles is in a range of2:1 to 100:1, preferably 3:1 to 40:1, preferably 4:1 to 36:1, preferably6:1 to 34:1, preferably 8:1 to 32:1, preferably 10:1 to 30:1, preferably12:1 to 28:1, preferably 14:1 to 26:1, preferably 16:1 to 24:1,preferably 18:1 to 22:1, or about 19:1. In certain embodiments, however,the weight ratio of the zinc oxide nanoparticles to the galliumoxynitride nanoparticles is less than 2:1 or greater than 100:1.

In one or more embodiments, the aforementioned nanoarchitecturedphotocatalytic material comprising the zinc oxide and gallium oxynitridenanoparticles is deposited onto a surface of a substrate. Thenanoarchitectured photocatalytic material may be adsorbed on the surface(e.g. by van der Waals and/or electrostatic forces) of the substrate.Exemplary substrates include fluorine doped tin oxide (FTO) film, indiumtin oxide (ITO) film, ITO coated polyethylene terephthalate (PET) film,a gold film, gold coated glass, aluminum oxide, titanium oxide, nickeloxide, tungsten oxide, strontium titanate, quartz, and silicon wafer. Ina preferred embodiment, the substrate is a metal oxide conductingsubstrate. In an even more preferred embodiment, the metal oxideconducting substrate used is fluorine doped tin oxide (FTO). Thesubstrate may be of any desirable shape, such as, a circle, a triangle,a rectangle, a pentagon, a hexagon, an irregular polygon, a circle, anoval, an ellipse, or a multilobe. Preferably the substrate isrectangular in shape with a length and width of 0.5-5 cm, 1-4 cm, or 2-3cm, respectively.

In a preferred embodiment, 70-100%, more preferably 80-99%, even morepreferably 85-97% of the surface of the metal oxide conducting substrateis covered with the nanoarchitectured photocatalytic material, though insome embodiments, less than 70% of the surface of the metal oxideconducting substrate is covered with the nanoarchitecturedphotocatalytic material.

As used herein, UV-vis spectroscopy or UV-vis spectrophotometry refersto absorption spectroscopy or reflectance spectroscopy in theultraviolet-visible spectral region. This means it uses light in thevisible and adjacent (near-UV and near-infrared) ranges. The absorptionor reflectance in the visible range directly affects the perceived colorof the chemicals involved. In this region of the electromagneticspectrum, molecules undergo electronic transitions. Molecules containingπ-electrons or non-bonding electrons (n-electrons) can absorb the energyin the form of ultraviolet or visible light to excite these electrons tohigher anti-bonding molecular orbitals. The more easily excited theelectrons (i.e. the lower the energy gap between the HOMO and the LUMO),the longer the wavelength of light it can absorb. This technique iscomplementary to fluorescence spectroscopy, in that fluorescence dealswith transitions from the excited state to the ground state, whileabsorption measures transitions from the ground state to the excitedstate. In one or more embodiments, the GaON/ZnO photoelectrode describedherein has an ultraviolet visible absorption with an absorption edge ina range of 410-520 nm, preferably 420-500 nm, preferably 430-490 nm,preferably 440-480 nm, preferably 450-470 nm, or about 466 nm. In someembodiments, the GaON/ZnO photoelectrode has an absorption edge at alonger wavelength relative to a ZnO deposited photoelectrode by at least30 nm, 40 nm, 50 nm, 52 nm, 54 nm, 56 nm, or at least 58 nm.

As used herein, band gap energy (E_(g)), band gap, and/or energy gaprefers to an energy range in a solid where no electron states can exist.In graphs of the electronic band structure of solids, the band gapgenerally refers to the energy difference (in electron volts) betweenthe top of the valence band and the bottom of the conduction band ininsulators and/or semiconductors. It is generally the energy required topromote a valence electron bound to an atom to become a conductionelectron, which is free to move within the crystal lattice and serve asa charge carrier to conduct electric current. Optoelectronic materialssuch as conjugated polymers are generally classified according to theirband gap, which is closely related to the HOMO/LUMO gap in chemistry.Band gap energies for the GaON/ZnO photoelectrode described herein maybe obtained using optical spectroscopies, e.g. UV-vis spectroscopyand/or electrochemical measurements, e.g. cyclic voltammetry (CV) anddifferential pulse voltammetry (DPV). In one or more embodiments, theGaON/ZnO photoelectrode of the present disclosure in any of itsembodiments has a band gap energy of 2.1-3.2 eV, 2.2-3.0 eV, 2.3-2.9 eV,2.4-2.8 eV, 2.5-2.7 eV, or about 2.58 eV. However, in some embodiments,the band gap energy may be less than 2.1 eV or greater than 3.2eVAccording to another aspect, the present disclosure relates to amethod of producing the GaON/ZnO photoelectrode of the first aspect. Themethod involves the steps of mixing zinc oxide nanoparticles and galliumoxynitride nanoparticles in a solvent to form a mixture. The mixture maycomprise zinc oxide nanorods at a concentration of 0.1-10 g/mL, 0.2-5g/mL, 0.3-4 g/mL, 0.4-3 g/mL, 0.5-2 g/mL, or 0.6-1 g/mL. In certainembodiments, 1 wt%, 3 wt%, or 5 wt% of GaON is present in the ZnOnanorods mixture. The zinc oxide nanoparticles used herein may havesizes, dimensions and properties as those previously described in thefirst aspect. In one or more embodiments, the zinc oxide nanoparticlesused herein are in the form of nanorods have an average diameter of20-200 nm, preferably 40-150 nm, more preferably 60-100 nm, and anaverage length of 500-2,000 nm, preferably 800-1,600 nm, more preferably1000-1,200 nm.

The mixture may comprise gallium oxynitride nanoparticles at aconcentration of 1-5,000 mg/mL, 10-4,000 mg/mL, 25-3,000 mg/mL, 50-2,000mg/mL, 100-1,000 mg/mL, 200-800 mg/mL, or 400-600 mg/mL. In certainembodiments, 1 wt%, 3 wt% or 5 wt% of GaON is present in ZnO nanorodsmixture, remianing 99 wt%, 97 wt%, or 95 wt% is ZnO nanorods. In apreferred embodiment, a weight ratio of the zinc oxide nanoparticles tothe gallium oxynitride nanoparticles is in a range of 2:1 to 100:1,preferably 3:1 to 40:1, 4:1 to 36:1, preferably 6:1 to 34:1, preferably8:1 to 32:1, preferably 10:1 to 30:1, preferably 12:1 to 28:1,preferably 14:1 to 26:1, preferably 16:1 to 24:1, preferably 18:1 to22:1, or about 19:1. The mixture may be agitated by an agitator, avortexer, a rotary shaker, a magnetic stirrer, a centrifugal mixer, oran overhead stirrer. In another embodiment, the mixture is left to stand(i.e. not agitated).

The gallium oxynitride nanoparticles used herein may have sizes,dimensions and properties as those previously described in the firstaspect. In one or more embodiments, the gallium oxynitride nanoparticlesused herein are in the form of nanosheets having an average thickness of5-80 nm, preferably 10-50 nm, more preferably 20-40 nm, and an averagelength of 200-1,500 nm, preferably 400-1,200 nm, more preferably600-1,000 nm. In one embodiment, the gallium oxynitride nanosheets maybe prepared using a sonochemical assisted solvothermal synthesis. Themethod involves mixing a gallium metal, an amino chelating agent (e.g.ethylenediamine), with water to form a GaON reaction mixture. In apreferred embodiment, the GaON reaction mixture may be formed bysonication using a sonication bath or a sonication probe for 0.1-4hours, 0.5-2 hours, or about 1 hour. The water may be mixed with thegallium metal and the amino chelating agent at the water addition rateof 1-20 mL/min, 2-10 mL/min, or about 5 mL/min. The GaON reactionmixture may then be heated at a temperature of 120-250° C. in anautoclave, a furnace, or an oven for 2-36 hours, 3-24 hours, or 6-12hours to form a GaON reaction precipitate. Collecting, washing, anddrying the GaON reaction precipitate to form the gallium oxynitridenanosheets. The GaON reaction precipitate may be collected viacentrifuge at 1000-5000 rpm, 2000-4000 rpm, or 2500-3500 rpm. The GaONreaction precipitate may be washed using a solvent such as acetone,ethanol, or both. The GaON reaction precipitate may then dried at atemperature of 75-150° C., 85-125° C., 95-110° C., or about 100° C. for0.5-6 hours, 1-4 hours, or about 2 hours in an oven or furnace.Preferably, the GaON reaction precipitate may be annealed at atemperature of 300-700° C., 400-600° C., or about 500° C. for 2-8 hours,3-6 hours, or about 4 hours thereby forming the gallium oxynitridenanosheets.

A specific embodiment of the preparation of the gallium oxynitridenanosheets includes but is not limited to the following:

For synthesis, few grams of Gallium metal are mixed with ethylenediamineand placed in an ultrasonic bath for an hour at 75° C. During theultrasonification, deionized water is added in 5 ml portions after every5 min. time span. The appearance of black suspension shows the formationof Ga— ethylenediamine complex. After complete dissolution of galliummetal in ethylenediamine water solution, the reaction mixture is shiftedinto a stainless steel autoclave containing a Teflon vessel. Thesolvothermal reaction is carried out for 3, 6, 12 and 24 hours at 180°C. respectively. Afterwards the reaction mixture containing precipitatesof Galliumoxynitride (GaON) in each case is centrifuged at 4000 RPM for5 mins, washed with ethanol and acetone respectively before drying in avacuum oven at 100° C. for two hours. Furthermore, the as prepared GaONnanosheets at 180° C. for 24 hours is further annealed at 500° C. for 4hours owing to its best morphology achieved for comparative PEC studies.

In an alternative embodiment, the mixture used for producing theGaON/ZnO photoelectrode may be formed by a one-pot procedure by addingthe zinc oxide nanorods directly to the GaON reaction precipitategenerated in the aforementioned method for the preparation of thegallium oxynitride nanosheets, without further collecting, washing,heating, or annealing the GaON reaction precipitate.

In a preferred embodiment, the solvent is water. The water may be tapwater, distilled water, bidistilled water, deionized water, deionizeddistilled water, reverse osmosis water, and/or some other water. In oneembodiment, the water is bidistilled to eliminate trace metals.Preferably the water is bidistilled, deionized, deinonized distilled, orreverse osmosis water and at 25° C. has a conductivity at less than 10µS·cm⁻¹, preferably less than 1 µS·cm⁻¹, a resistivity greater than 0.1MΩ·cm, preferably greater than 1 MΩ·cm, more preferably greater than 10MΩ·cm, a total solid concentration less than 5 mg/kg, preferably lessthan 1 mg/kg, and a total organic carbon concentration less than 1000µg/L, preferably less than 200 µg/L, more preferably less than 50 µg/L.In an alternative embodiment, other solvents such as methanol, ethanol,i-propanol, and n-butanol may be used in addition to, or in lieu of thewater. The mixture may be sonicated for 0.1-4 hours, preferably 0.5-3hours, more preferably 1-2 hours, using a sonication bath or asonication probe to form a dispersed mixture. Alternatively, the mixturemay not be sonicated but instead mixed, stirred, shaken, blended, and/oragitated for an equivalent amount time. In an alternative embodiment,the mixture may only be mixed to form a homogeneously dispersed mixture,and then left to stand for the previously indicated amount of time.

In a preferred embodiment, a deposited substrate may be formed byvertically suspending a metal oxide conducting substrate (e.g. FTO)inside the dispersed mixture for 2-12 hours, 4-10 hours, 5-8 hours, orabout 6 hours at a pressure of 100-300 kPa, 125-275 kPa, 150-250 kPa,175-225 kPa, or 190-200 kPa. The depositing may be performed within afurnace, an oven, or an autoclave. Preferably the depositing processstarts at room temperature or 20-50° C., and then the temperature isincreased to a target temperature of 50-120° C., 60-100° C., 70-90° C.,or about 80° C. at a rate of 2-15° C./min, preferably 4-10° C./min, orabout 5° C./min. The depositing process may continue at the targettemperature for 0.5-12 hours, 1-10 hours, 2-8 hours, 4-7 hours, or about6 hours. In a preferred embodiment, the suspending of the metal oxideconducting substrate (e.g. FTO) inside the dispersed mixture may notinvolve any forms of agitation (e.g. the substrate is left to stand inthe dispersed mixture through the depositing process). Alternatively,the depositing may involve shaking, stirring, sonicating, and/orbubbling the dispersed mixture during the depositing process. In analternative embodiment, the dispersed mixture may be deposited onto asurface of the metal oxide conducting substrate by other applicationprocedures including, but not limited to, drop-casting, spin-coating,spraying, and spreading methods.

The deposited substrate may be left at room temperature and washed witha solvent such as water and/or ethanol. The deposited substrate may thenbe heated at a temperature of 70-150° C., 80-120° C., 90-110° C., orabout 100° C. for 0.5-12 hours, 1-6 hours, or 2-4 hours. In one or moreembodiments, the heating is followed by cooling to a temperature rangingfrom 5-40° C., 10-30° C., 15-25° C., or room temperature thereby formingthe GaON/ZnO photoelectrode.

According to another aspect, the present disclosure relates to aphotoelectrochemical cell containing the GaON/ZnO photoelectrode of thefirst aspect in any of its embodiment, a counter electrode, and anelectrolyte solution comprising water and an inorganic salt in contactwith both the GaON/ZnO photoelectrode and the counter electrode. As usedherein, the GaON/ZnO photoelectrode may be considered as a workingelectrode in the photoelectrochemical cell.

In one embodiment, the photoelectrochemical cell is a vessel having aninternal cavity for holding the electrolyte solution. The vessel may becylindrical, cuboid, frustoconical, spherical, or some other shape. Thevessel walls may comprise a material including, but not limited to,glass, quartz, polypropylene, polyvinyl chloride, polyethylene, and/orpolytetrafluoroethylene. In a preferred embodiment, a vessel with atransparent window is used. For example, the window may comprise glassor quartz, though in one embodiment, a polymeric material transparent tovisible light and chemically stable with the reaction mixture may beused. As defined herein, “transparent” refers to an optical quality of acompound wherein a certain wavelength or range of wavelengths of lightmay traverse through a portion of the compound with a small loss oflight intensity. Here, the “transparent window” may causes a loss ofless than 10%, preferably less than 5%, more preferably less than 2% ofthe intensity of a certain wavelength or range of wavelengths of light.In one embodiment, the vessel wall and window may comprise the samematerial, for example, a vessel may comprise quartz walls, which mayalso function as transparent windows. The internal cavity may have avolume of 2 - 100 mL, preferably 2.5 - 50 mL, more preferably 3 - 20 mL.In another embodiment, the internal cavity may have a volume of 100 mL -50 L, preferably 1 - 20 L, more preferably 2 - 10 L. In anotherembodiment, for instance, for pilot plant testing, the internal cavitymay have a volume of 50 - 10,000 L, preferably 70 - 1,000 L, morepreferably 80 - 2,000 L. In another embodiment, the internal cavity mayhave a volume larger than 2,000 L. In one embodiment, one or morephotoelectrochemical cells may be connected to each other in paralleland/or in series. In another embodiment, the electrolyte solution may bein contact with more than one working electrode and/or more than onecounter electrode.

In one embodiment, the counter electrode comprises platinum, gold,silver, or carbon. In a preferred embodiment, the counter electrodecomprises platinum. In one embodiment, the counter electrode may be inthe form of a wire, a rod, a cylinder, a tube, a scroll, a sheet, apiece of foil, a woven mesh, a perforated sheet, or a brush. The counterelectrode may be polished in order to reduce surface roughness or may betexturized with grooves, channels, divots, microstructures, ornanostructures. In at least one embodiment, where the counter electrodecomprises platinum, the counter electrode is in the form of a wire. Insome embodiments, the counter electrode may comprise some otherelectrically-conductive material such as gold, platinum-iridium alloy,iridium, titanium, titanium alloy, stainless steel, and cobalt alloy. Asdefined herein, an “electrically-conductive material” is a substancewith an electrical resistivity of at most 10⁻⁶ Ω·m, preferably at most10⁻⁷ Ω·m, more preferably at most 10⁻⁸ Ω·m at a temperature of 20 - 25°C. In a preferred embodiment, the counter electrode has at least oneouter surface comprising an essentially inert, electrically conductingmaterial, such as platinum, gold, silver, or carbon. In anotherpreferred embodiment, the counter electrode may comprise solid platinum,gold, or carbon. The material of the counter electrode should besufficiently inert to withstand the chemical conditions in theelectrolyte solution without substantially degrading during aphotoelectrochemical reaction.

In one embodiment, the electrolyte solution has an inorganic saltconcentration of 0.05-1 M, preferably 0.1-0.8 M, preferably 0.2-0.7 M,preferably 0.4-0.6 M, or about 0.5 M, though in some embodiments, theinorganic salt may be present at a concentration less than 0.05 M orgreater than 1 M. The inorganic salt may be Na₂SO₄, K₂SO₄, ZnSO₄, LiCl,NaCl, KCl, NaClO₄, KNO₃, NaNO₃, NH₄Cl, NH₄NO₃, LiPF₆, MgCl₂, or someother inorganic salt. Preferably the inorganic salt is Na₂SO₄. In analternative embodiment, an inorganic acid such as HCl, HClO₄, HNO₃, orH₂SO₄ may be used in addition to, or in lieu of the inorganic salt. Inanother alternative embodiment, an inorganic base such as LiOH, NaOH,KOH, or NH₃ may be may be used in addition to, or in lieu of theinorganic salt. In one or more embodiments, the electrolyte solution hasa pH in a range of 5-9, preferably 6-8, more preferably a pH at about 7.In an alternative embodiment, the electrolyte solution has a pH lessthan 5 or greater than 9.

In one or more embodiments, the GaON/ZnO photoelectrode has aphoto-current density in a range of 0.01-2 mA/cm², preferably 0.1-1.8mA/cm², preferably 0.2-1.6 mA/cm², preferably 0.4-1.4 mA/cm², preferably0.6-1.2 mA/cm², preferably 0.8-1 mA/cm² when the photoelectrochemicalcell is subjected to a potential of 0.1 to 1.5 V, preferably 0.5 to 1.4V, more preferably 0.8 to 1.3 V, or about 1.23 V under visible lightirradiation.

In one embodiment, the GaON/ZnO photoelectrode has a photo-currentdensity greater than 1 mA/cm² when the photoelectrochemical cell issubjected to a potential greater than 0.9 V under visible lightirradiation. In one or more embodiments, the GaON/ZnO photoelectrode hasa photo-current density in a range of 1-2 mA/cm², 1.1-1.8 mA/cm²,1.2-1.7 mA/cm², 1.3-1.6 mA/cm², or 1.4-1.5 mA/cm², when thephotoelectrochemical cell is subjected to a potential of 0.9 to 2 V, 1.0to 1.8 V, 1.2 to 1.5 V, or 1.3 to 1.4 V under visible light irradiation.In some embodiments, the photoelectrode has a photo-current density lessthan 0.2 mA/cm² when the photoelectrochemical cell is subjected to apotential less than 0.5 V under visible light irradiation. In analternative embodiment, the photoelectrode has a photo-current densityof 0.01-0.2 mA/cm², 0.02-0.15 mA/cm², 0.04-0.1 mA/cm², or 0.06-0.08mA/cm² when the photoelectrochemical cell is subjected to a potential of0.1 to 1.5 V, preferably 0.5 to 1.4 V, more preferably 0.8 to 1.3 V, orabout 1.23 V under dark (without irradiation). In some embodiments, thephoto-current density of the GaON/ZnO photoelectrode under dark is atleast 25% less, at least 40% less, at least 50% less, at least 60% less,at least 70% less, at least 80% less, at least 90% less, or at least 95%less than that of the GaON/ZnO photoelectrode under visible lightirradiation.

In certain embodiments, visible light irradiation may be performed by alight source internal or external to the photoelectrochemical cell andmay provide the photon energy necessary to activate the photocatalyticmaterial of the photoelectrochemical cell in any of its embodiments. Thelight source may be any known light source including, but not limitedto, natural solar sunlight, simulated solar light, UV light, laserlight, incandescent light, and the like. Exemplary light sourcesinclude, but are not limited to, a xenon lamp such as a xenon arc lampand a xenon flash lamp, a mercurial lamp, a metal halide lamp, an LEDlamp, a solar simulator, and a halogen lamp. In certain embodiments, twoor more light sources may be used. In a preferred embodiment, asimulated solar light may be used as the light source. In anotherpreferred embodiment, natural sunlight may be used as the light source.The light may be visible light having a wavelength of 400-800 nm,preferably 420-700 nm, more preferably 450-600 nm. The light source maycomprise one or more wavelengths within the range of 400-800 nm.Preferably a light source is used which emits a broad wavelength rangeof light and which comprises a portion or the entire visible lightspectrum. A light source may additionally emit light of wavelengthsbelow 400 nm and/or above 800 nm. In one embodiment, a filter may beused to prevent UV light from entering the reaction mixture, forexample, a filter that blocks light with wavelengths less than 420 nmmay be used with a simulated solar light, xenon, or a mercury gasdischarge lamp. Alternatively, a light source may be used which onlyemits light within the visible spectrum. In an alternative embodiment,the GaON/ZnO photoelectrode may be irradiated with UV light, with orwithout visible light. The light source may emit a total power of50-2000 W, preferably 100-1500 W, more preferably 500-1000 W, and may bepositioned 2-30 cm, preferably 5-20 cm, more preferably 8-15 cm from theclosest surface of the photoelectrode. In a preferred embodiment, thelight source has an intensity of 500-4000 W/m², preferably 700-2000W/m², more preferably 900-1500 W/m², or about 1000 W/m² (100 mW/cm², 1SUN power).

In one embodiment, the photoelectrochemical cell further comprises areference electrode in contact with the electrolyte solution. Areference electrode is an electrode which has a stable and well-definedelectrode potential. The high stability of the electrode potential isusually reached by employing a redox system with constant (buffered orsaturated) concentrations of each relevant species of the redoxreaction. A reference electrode may enable a potentiostat to deliver astable voltage to the working electrode or the counter electrode. Thereference electrode may be a saturated calomel electrode (SCE), astandard hydrogen electrode (SHE), a normal hydrogen electrode (NHE), areversible hydrogen electrode (RHE), a copper-copper(II) sulfateelectrode (CSE), a silver chloride electrode (Ag/AgCl), a pH-electrode,a palladium-hydrogen electrode, a dynamic hydrogen electrode (DHE), amercury-mercurous sulfate electrode, or some other type of electrode. Ina preferred embodiment, a reference electrode is present and is asaturated calomel electrode (SCE). However, in certain embodiments, thephotoelectrochemical cell does not comprise a reference electrode.

In one or more embodiments, the photo-current density of the GaON/ZnOphotoelectrode decreases by less than 15%, preferably less than 12%,preferably less than 10%, preferably less than 8%, preferably less than6%, preferably less than 5%, preferably less than 4%, preferably lessthan 3%, preferably less than 2%, preferably less than 1% aftersubjecting the photoelectrochemical cell to a potential of 0.1 to 1.5 V,0.25 to 1.25 V, 0.5 to 1 V, or 0.6-0.8 V under visible light irradiationfor about 50-2,000 minutes, about 100-1.000 minutes, about 200-800minutes, or about 400-600 minutes relative to that measured immediatelyafter the subjecting commences.

According to another aspect, the present disclosure relates to a methodof splitting water into hydrogen gas and oxygen gas. The method involvesthe steps of subjecting the photoelectrochemical cell of the secondaspect to a potential of 0.5 to 2.0 V, preferably 0.6-1.8 V, preferably0.7-1.6 V, preferably 0.8-1.4 V, preferably 0.9-1.3 V, preferably 1-1.2V, and concurrently irradiating the photoelectrochemical cell withvisible light, thereby forming hydrogen gas and oxygen gas. In someembodiments, the photoelectrochemical cell is subjected to a potentialless than 0.5 V or greater than 2.0 V.

Photoelectrochemical water splitting dissociates water into itsconstituent parts, hydrogen (H₂) and oxygen (O₂), by applying apotential to a photoelectrochemical cell under either artificial ornatural light. The process generally involve a photoelectrocatalystabsorbing a photon with sufficient energy (above 1.23 eV, λ < ~1000 nm),subsequently permitting photoexcited electrons and holes to separate andmigrate to the surface of the photoelectrocatalyst material, reducingadsorbed species (i.e. water). Two types of photochemical systemsoperate via photocatalysis. One uses semiconductor surfaces ascatalysts. In these devices the semiconductor surface absorbs solarenergy and acts as an electrode for water splitting. The othermethodology uses in-solution metal complexes as catalysts.

In one embodiment, the visible light may have a wavelength of 400-800nm, preferably 420-700 nm, more preferably 450-600 nm. Irradiating thephotoelectrochemical cell with visible light during water splitting maybe performed by the aforementioned light source internal or external tothe photoelectrochemical cell. The light source may comprise one or morewavelengths within the range of 400-800 nm. The light source mayadditionally emit light of wavelengths below 400 nm and/or above 800 nm.For example, a simulated solar light may be used as the light source.For another example, natural sunlight may be used as the light source.The light source may emit a total power of 50-2000 W, preferably100-1500 W, more preferably 500-1000 W.

In one embodiment, the electrolyte solution of the photoelectrochemicalcell during water splitting has a pH in a range of 4-10, preferably 5-9,preferably 6-8, more preferably a pH at about 7. In an alternativeembodiment, the electrolyte solution has a pH less than 4 or greaterthan 10 during water splitting. In one embodiment, the aforementionedmethod of splitting water into hydrogen gas and oxygen gas involvessubjecting the photoelectrochemical cell to a potential of 0.1-2.5 V,preferably 0.25-2.0 V, preferably 0.4-1.5 V, preferably 0.6-1.25 V,preferably 0.8-1.0 V, and concurrently irradiating thephotoelectrochemical cell with visible light for 0.1-24 hours, 0.5-18hours, 1-12 hours, 2-11 hours, 3-10 hours, 4-9 hours, 5-8 hours, or 6-7hours.

Preferably, the counter electrode (e.g. the platinum wire) functions asthe photocathode by receiving a negative potential to reduce water intoH₂ gas and OH⁻, while the GaON/ZnO photoelectrode functions as thephotoanode by receiving a positive potential to oxidize OH⁻ into O₂ gasand H₂O.

In one embodiment, the method further comprises a step of separatelycollecting H₂-enriched gas and O₂-enriched gas. In one embodiment, thespace above each electrode may be confined to a vessel in order toreceive or store the produced gases from one or both electrodes. Thecollected gas may be further processed, filtered, or compressed.Preferably the H₂-enriched gas is collected above the photocathode, andthe O₂-enriched gas is collected above the photoanode. In oneembodiment, the H₂-enriched gas and the O₂-enriched gas are not 100 vol%H₂ and 100 vol% O₂, respectively. For example, the enriched gas may alsocomprise N₂ from air, and water vapor and other dissolved gases from theelectrolyte solution. The H₂-enriched gas may also comprise O₂ from air.The H₂-enriched gas may comprise greater than 20 vol% H₂, preferablygreater than 40 vol% H₂, more preferably greater than 60 vol% H₂, evenmore preferably greater than 80 vol% H₂, relative to a total volume ofthe receptacle collecting the produced H₂ gas. The O₂-enriched gas maycomprise greater than 20 vol% O₂, preferably greater than 40 vol% O₂,more preferably greater than 60 vol% O₂, even more preferably greaterthan 80 vol% O₂, relative to a total volume of the receptacle collectingthe produced O₂ gas. In some embodiments, the produced gases may bebubbled into a vessel comprising water or some other liquid, and ahigher concentration of H₂ or O₂ may be collected. In one embodiment,produced O₂ and H₂, or H₂-enriched gas and O₂-enriched gas may becollected in the same vessel.

The examples below are intended to further illustrate protocols forpreparing, characterizing the GaON/ZnO photoelectrode,photoelectrochemical cell, and uses thereof, and are not intended tolimit the scope of the claims.

Example 1 Materials and Methods

The raw materials i.e. GaON nanosheets and ZnO nanorods (NRs) wereprepared via established synthetic routes [N. Iqbal, I. Khan, Z.H.Yamani, A. Qurashi, Sonochemical assisted solvothermal synthesis ofgallium oxynitride nanosheets and their solar-drivenphotoelectrochemical water-splitting applications, Sci. Rep. 6 (2016)32319; and Q. Ahsanulhaq, J.H. Kim, Y.B. Hahn, Controlled selectivegrowth of ZnO nanorod arrays and their field emission properties,Nanotechnology, vol. 18 (48), p. 485307, each incorporated herein byreference in their entirety]. One pot GaON/ZnO nanoarchitecturesynthesis was achieved by mixing desirable concentrations of GaONnanosheets and ZnO NRs by weight (5:95) in a beaker containing 50 mL DIwater through regular sonication for 1 hr. The uniform mixture was thentransferred to a Teflon vessel with vertically suspended, freshly washed2 × 2 cm² FTO glass (purchased from Sigma Aldrich). The purpose ofsuspending FTO in the reaction mixture was to deposit the as-synthesizedGaON/ZnO nanoarchitecture directly over uncovered conducting surface ofFTO to develop a homogenous film, which might possibly boast the deviceperformance. The Teflon was sealed in the autoclave and kept at 80° C.for 6 hrs in a laboratory oven. The temperature was ramped at a rampingrate of 5° C./min to avoid vigorous heating and to achieve uniformreaction rate. A uniform firm, thin film of GaON/ZnO nanoarchitecturewas deposited on the FTO glass within the autoclave at an elevatedpressure (FIG. 2 ). The autoclave was replaced from the oven and wascooled spontaneously. The GaON/ ZnO deposited FTO electrode was removedfrom autoclave, washed gently with DI water and kept at 100° C. forfurther drying for 2 hrs. For comparison purpose, ZnO/FTO and GaON/FTOphotoelectrodes were also prepared by a similar approach.

The crystallinity of GaON/ZnO nanoarchitecture was examined byMini-X-ray Diffraction (Mini-XRD) with Cu Ka X-ray radiation source (k =0.15406 nm). The surface morphology and composition were identified byfield emission scanning electron microscopy (FE-SEM) (Tescan Lyra-3))and energy dispersion X-ray spectroscopy (EDX) equipped with FE-SEM,respectively. Vibrational spectroscopic information was obtained viaInfrared (FT-IR) spectroscopy (measured at room temperature with theThermo-Fisher device from 600 to 1200 cm⁻¹ region). The XPS analysis wasperformed via V.G. Scientific ESCALAB Mk(II) spectrometer using anon-monochromatic Al source (Ka, 1486.6 eV). The output power wasadjusted to 130 W. Before starting, the binding energy (eV) of thedevice was calibrated with standard reference lines (Cu 2p3/2 = 932.67eV, Cu 3p3/2 = 74.9 eV and Au 4f7/2 = 83.98 eV). The permitted energy ofthe electron analyzer was fixed at 10 eV. The energy resolution wasabout 1.0 eV. Diffuse Reflectance Spectroscopy (DRS) (HORIBA) andPhotoluminescence (PL) were utilized to scrutinize optical properties ofthe pristine ZnO, GaON and their nanoarchitecture.

The photoelectrochemical water splitting performance was examined usinga three-electrode photochemical cell supported by a potentiostat(autolab) and solar simulator (ORIEL SOL-3A). The intensity of theartificial light source was calibrated with standard silicon diode(ORIEL) photodevice and tuned at 1 SUN (100 mW/cm²) power. The solarsimulator was provided with 1.5A.M and equipped with UV cut offwavelength (< 420 nm) filters. The three-electrode cell containedPlatinum (Pt) as a counter electrode, Secondary Calomel Electrode (SCE)as a reference electrode and GaON, ZnO or GaON/ZnO nanoarchitecturephotoelectrode as a working electrode. Moreover, 0.5 M sodium sulfate(Na₂SO₄) solution in DI water was used as an electrolyte forphotocurrent density measurements. The scan rate was maintained at 0.05V/s for Linear Sweep Voltammetry (LSV) between 1.5 V and - 1.5 V. Thechronoamperometric study was performed at the chopping ON/OFF rate of ~60 s/cycle.

DFT calculations were conducted using the generalized gradientapproximation of Perdew-Burke-Emzerhof for the exchange-correlationenergy [J.P. Perdew, K. Burke, M. Emzerhof, Generalized gradientapproximation made simple, Phys. Rev. Lett. 77 (1996) 3865-3868,incorporated herein by reference in its entirety]. The Brillouin zonesampling was done using 12 × 12 × 12 Monkhorst-Pack k-point sampling forthe unit cell of GaON (see FIG. 1A). For the slab geometries (FIGS. 1Band 1C) 12 × 12 × 1 k-points were used due to the presence of vacuumspacing more than 10 ̊Å perpendicular to the slabs. The electrostaticpotential was determined on a real-space grid with mesh cutoff energy of150 Ry and double-zeta-polarized basis sets of numerical orbitals wereused to describe the atoms. Van der Waals interactions were introducedby Grimme’s empirical dispersion correction [S. Grimme, SemiempiricalGGA-type density functional constructed with a long-range dispersioncorrection, J. Comput. Chem. 27 (2006) 1787-1799, incorporated herein byreference in its entirety]. The convergence criterion was 0.01 eV/Å forHellman-Feynman forces. The adsorption energy of a water molecule isdefined as

$\begin{matrix}{E_{\text{ads}}\mspace{6mu} = \mspace{6mu} E_{\text{GaON}/{{(\text{GaON+ZnO})}\text{+H2O}}}\mspace{6mu} - \mspace{6mu} E_{\text{GaON}/{(\text{GaON+ZnO})}}\mspace{6mu} - E_{\text{H2O}},} & \text{­­­(equati on 1)}\end{matrix}$

where E_(GaON/(GaON+ZnO))_(+H2O) is the total energy of GaON/(GaON+ZnO)slab with an adsorbed water molecule, E_(GaON/(GaON+ZnO)) is the overallenergy of GaON/(GaON+ZnO) slab and E_(H2O) is the overall energy ofisolated water molecule. All calculations were conducted using thecomputational package Atomistix toolkit [distributed by QuantumWisecompany, Copenhagen, Denmark.http://www.quantumwise.com].

Example 2 XRD Analysis

The XRD investigation was useful to understand the crystal phase of thefinal product, as crystallinity and crystal phase distinctly effect theoverall performance of the photoelectrodes [C.S. Chua, D. Ansovini,C.J.J. Lee, Y.T. Teng, L.T. Ong, D. Chi, T.S.A. Hor, R. Raja, Y.-F. Lim,The effect of crystallinity on photocatalytic performance of Co₃O₄water-splitting cocatalysts, Phys. Chem. Chem. Phys. 18 (2016)5172-5178, incorporated herein by reference in its entirety]. FIG. 3represents the stacked XRD patterns for GaON, ZnO and GaON/ZnOnanoarchitectures, respectively. The XRD 2θ values for FTO have beensubtracted from all results to obtain real XRD patterns for ZnO, GAONand GaON/ZnO nanoarchitecture, respectively. Characteristic intense XRDpeak [2 2 2] for ZnO was observed at 2θ = ~ 34° in pristine as well asin GaON/ZnO nanoarchitecture XRD pattern with a slightly lower intensity[H.R. Pant, B. Pant, R.K. Sharma, A. Amarjargal, H.J. Kim, C.H. Park,L.D. Tijing, C.S. Kim, Antibacterial and photocatalytic properties ofAg/TiO₂/ZnO nano-flowers prepared by facile one-pot hydrothermalprocess, Ceram. Int. 39 (2013) 1503-1510, incorporated herein byreference in its entirety]. In addition, other peaks for ZnO alsoappeared in the nanoarchitecture spectrum at almost the same level aspristine ZnO. Several peaks for GaON could be seen in the GaON/ZnOnanoarchitecture XRD spectrum from 33° to 43°. Some additional peakswere also observed in the nanoarchitecture spectrum, especially at thefar end of the XRD spectrum at 44.76°, 59°, 67° and 69.48°. These peakswhich were not observed in pristine ZnO and GaON spectra might be due toformation of GaON/ZnO architecture.

Example 3 SEM Analysis

FE-SEM microscopic details were obtained for GaON and GaON/ ZnOnanoarchitecture. FIGS. 4A and 4B each represent the low and highmagnification FE-SEM images for GaON nanosheets. The nanoflowers sheetsmorphology with variable sizes and channels like appearance was observedfor GaON. Moreover, FIGS. 4C and 4D show the GaON/ZnO nanoarchitecturedistribution at lower and higher resolution. The images demonstrated aclear intercalation of the two materials at the nanoscale level. The ZnONRs could be seen bulging out from GaON nanosheets in the micrographs inan interposing pattern.

Example 4 EDX Analysis

The EDX and elemental mapping in FIG. 4 detected the presence ofnitrogen in GaON. These Figures also provided further details about thenanoarchitecture comprising GaON nanosheets and ZnO nanorods. EDXspectrum in FIG. 5A displayed known peaks for Zn, Ga, O and N. Thequantitative ratio indicated that nitrogen existed in a significantamount (~ 3%) in the final product. Mapping results in FIGS. 5B-Esupported the EDX results and mapped area showed a significantdistribution of nitrogen entities in the nanoarchitecture. Othernitrogen-doped materials were extensively studied for water splittingapplications [C.K. Chen, Y. Shen, H.M. Chen, C. Chen, T. Chan, J. Lee,R. Liu, Quantum-dot-sensitized nitrogen-doped ZnO for efficientphotoelectrochemical water splitting, Eur. J. Inorg. Chem. 2014 (2014)773-779, incorporated herein by reference in its entirety].

Example 5 XPS Analysis

XPS profiling was carried out to explore the nature of bonding and todetect the existence of nitrogen in the GaON/ZnO product. The core levelfitted/deconvoluted XPS spectra (blue lines) of the elemental componentsof GaON/ZnO were indicated in FIGS. 6A-E. FIG. 6A showed broader fittedXPS binding energy peak for nitrogen (1 s) lies from 390 to 402 eV. Thefitted peaks covered three sub-peaks, which can be assigned to Ga-O(394.6 eV), Ga-N (397.5 eV) and Ga-O-N (399.9 eV) bonding aftercomparing with available literature [K. Maeda, T. Takata, M. Hara, N.Saito, Y. Inoue, H. Kobayashi, K. Domen, GaN:ZnO solid solution as aphotocatalyst for visible-light-driven overall water splitting, J. Am.Chem. Soc. 127 (2005) 8286-8287; and Z. Wang, B. Huang, L. Yu, Y. Dai,P. Wang, X. Qin, X. Zhang, J. Wei, J. Zhan, X. Jing, H. Liu, M.-H.Whangbo, Enhanced ferromagnetism and tunable saturation magnetization ofMn/C-codoped GaN nanostructures synthesized by carbothermal nitridation,J. Am. Chem. Soc. 130 (2008) 16366-16373, each incorporated herein byreference in their entirety]. The characteristic oxygen (1 s) XPS peakwas observed at 528-535 eV and deconvolution showed two sub-peaks. Thesepeaks might be assigned to Zn-O (530.66 eV) and Ga-O-N (532.3 eV) bonds(FIG. 6B) [W. Wei, Z. Qin, S. Fan, Z. Li, K. Shi, Q. Zhu, G. Zhang,Valence band offset of β-Ga₂O₃/wurtzite GaN heterostructure measured byX-ray photoelectron spectroscopy, Nanoscale Res. Lett. 7 (2012) 562,incorporated herein by reference in its entirety]. The observed Zn(2p_(½)) and (2p_(3/2)) distinctive binding peaks at 1022 and 1045 eVwere revealed in FIG. 6C. These peaks substantiated that the Zn existedin the Zn²⁺ state in GaON/ZnO nanoarchitecture, thus the oxidation stateof Zn remained unchanged [S.J. Wilkins, T. Paskova, A. 1vanisevic,Modified surface chemistry, potential, and optical properties of polargallium nitride via long chained phosphonic acids, Appl. Surf. Sci. 327(2015) 498-503, incorporated herein by reference in its entirety]. TheGallium elemental XPS peaks for Ga (3d) and (2p_(3/2)) were observed in16-24 eV and 1012-1022 eV regions, respectively (see FIGS. 6D and 6E).The peaks resolved into three sub-peaks i.e., the red and greenhighlighted areas under that were consigned to Ga—N and Ga—O bondingcorrespondingly, the black highlighted peaks represented some of thetraces of unreacted Ga metal.

Example 6 FT-IR Analysis

Comparative FT-IR results can be helpful to observe the variations inthe vibrational properties of GaON/ZnO from pristine GaON nanosheets andZnO NRs. The stacked IR spectra for GaON, ZnO and GaON/ZnO NRAs,respectively, were provided in FIG. 7 . The peaks appeared in ZnOspectrum at 925 cm⁻¹ and 883 cm⁻¹ were reported for Zn-O vibrationalstretching due to Zn-O lattice. These peaks were slightly shiftedtowards higher frequencies in GaON/ZnO spectrum to 921 cm⁻¹ and 880cm⁻¹. These significant shifts could be associated to the generation ofnew bonds in the GaON/ZnO [H.R. Pant, B. Pant, R.K. Sharma, A.Amarjargal, H.J. Kim, C.H. Park, L.D. Tijing, C.S. Kim, Antibacterialand photocatalytic properties of Ag/TiO₂/ZnO nano-flowers prepared byfacile one-pot hydrothermal process, Ceram. Int. 39 (2013) 1503-1510,incorporated herein by reference in its entirety]. By looking to theindexed values in GaON/ZnO spectrum, it could be observed that thespectral vibrational peaks of GaON also appeared at slightly shiftedpositions compared to the original GaON spectrum by 3-5 cm⁻¹ at 1098,985, 858, 745 cm⁻¹ as well as a broadened peak around 665 cm⁻¹. Thisprovided an evidence for the formation of the nanoarchitecture. Thephotoactive GaON/ZnO spectrum also showed new peaks, which appeared dueto the mixing of GaON and ZnO vibrational modes. The peaks appeared at1025, 921, 811 and 690 cm⁻¹ were assigned to GaON/ZnO composite.

Example 7 Optical Properties

Optical properties are of considerable importance as they providecrucial understanding on bandgap, which is necessary for assessingsolar-driven water splitting performance [H. Yan, X. Wang, M. Yao, X.Yao, Band structure design of semiconductors for enhanced photocatalyticactivity: the case of TiO₂, Prog. Nat. Sci. Mater. Int 23 (2013)402-407, incorporated herein by reference in its entirety]. One canpredict bandgap values based on the absorption edge. UV/Vis-DRS spectrain FIG. 8A showed the effect of GaON on ZnO absorption edge. The ZnOspectrum indicated a redshift towards higher wavelength in the visibleregion with GaON loading. The approximate absorption edges for ZnO,GaON/ZnO, and GaON were found at 408, 465 and 530 nm, which correspondedto 3.26, 2.58 and 2.01 eV bandgap, respectively, (FIG. 8B) usingKubelka-Munk equation [R. Beranek, H. Kisch, Tuning the optical andphotoelectrochemical properties of surface-modified TiO₂, Photochem.Photobiol. Sci. 7 (2008) 40-48, incorporated herein by reference in itsentirety]. Materials with a small bandgap may be useful forphotocatalytic applications, as these materials can efficiently absorbthe visible light region of electromagnetic radiation [M.W. Shah, Y.Zhu, X. Fan, J. Zhao, Y. Li, Facile synthesis of defective TiO_(2-x)nanocrystals with high surface area and tailoring bandgap forvisible-light photocatalysis, Nat. Publ. Gr. (2015) 1-8, incorporatedherein by reference in its entirety]. For water splitting reaction, theappropriate bandgap is determined to be around 2 eV [A.J. Nozik, Chapter13. Novel Approaches to Water Splitting by Solar Photons, (2013), pp.359-388, incorporated herein by reference in its entirety]. The decreasein the bandgap of GaON/ZnO nanoarchitecture could be ascribed to theintermixing of the bandgap levels of higher bandgap ZnO (3.24 eV) withlower bandgap GaON (2.01 eV). Both of these materials advanced theirvalance and conductance band in the GaON/ZnO nanoarchitecture [K.M.Domen, Particulate Oxynitrides for Photocatalytic Water Splitting UnderVisible Light, 2013, pp. 109-131, incorporated herein by reference inits entirety]. The lowest bandgap, i.e. 2.58 eV in case of GaON/ZnO issuitable for photoelectrochemical water splitting applications.

Additionally, photoluminescence studies also provide essentialinformation about the lifetime and recombination of photogeneratedexcitons [T. Tachikawa, T. Ochi, Y. Kobori, Crystal-face-dependentcharge dynamics on a BiVO₄ photocatalyst revealed by single-particlespectroelectrochemistry, ACS Catal. 6 (2016) 2250-2256, incorporatedherein by reference in its entirety]. The photoluminescence (PL)spectrum of ZnO and GaON/ZnO were indicated in FIG. 8C, which distinctlyshowed substantial quenching of ZnO by an almost double fold innanoarchitecture with GaON, with wavelength shifting from 383 nm to 406nm. The PL quenching had a direct effect on the overall lifetime ofexcitons and recombination rates, and hence the spectrum showed animproved lifetime for GaON/ZnO excitons compared to pristine ZnO. Theshift towards higher wavelength indicated that GaON was effectivelyembedded in the structure and therefore affected the overall opticalproperties [J.-H. Park, A. Mandal, S. Kang, U. Chatterjee, J.S. Kim,B.-G. Park, M.-D. Kim, K.-U. Jeong, C.-R. Lee, Hydrogen generation usingnon-polar coaxial InGaN/GaN multiple quantum well structure formed onhollow n-GaN nanowires, Sci. Rep. 6 (2016) 31996, incorporated herein byreference in its entirety].

Example 8 Photoelectrochemical Properties

The photoelectrochemical results were collected in the form of LSV andchronoamperometry graphs. FIG. 9A presented the LSV results, whichclearly indicated no significant dark current at first instant asindicated by the dotted lines. Under electromagnetic illumination, thecurrent densities jumped considerably as indicated by the solid lines.As the voltage increased, the current density values increasedaccordingly. A significant elevation in current density could beobserved for GaON/ZnO nanoarchitectures at onset potential of 0.6 V ascompare to pristine GaON and ZnO and reached to saturation at 1.1 V. Thecomparative photocurrent values for pristine GaON, pristine ZnO, andGaON/ ZnO were measured to be ~ 0.2, 0.45, and 1.2 mA/cm², respectively,at 1.2 V bias potential. Thus, it was clear that GaON improved thephotocurrent of ZnO from 0.45 to 1.2 mA/cm², which was more than 2-foldsincrease under the light. FIG. 9B showed the periodic chronoamperometricphotocurrent results of GaON, ZnO and GaON/ ZnO at 1.0 V bias voltageunder light and dark. The light was provided for ~ 60 s, followed by thedark interval of ~ 60 s. These results are in accordance with the LSVmeasurements and a significant increase in current densities wasobserved under the light. The GaON/ZnO nanoarchitecture showed a maximumjump at 0.95-1.0 mA/cm² in almost all cycles, which is about 2.4 timeshigher than ZnO (~ 0.42 mA) and several times higher than GaON (0.018mA). These results also revealed that even after many cycles, thenanoarchitecture showed good stability. FIG. 9C represents the stabilitymeasurement for the materials. The stability line remained almost stableafter several minutes and the current density for GaON/ZnOnanoarchitecture only dropped from 0.97 to 0.96 mA/cm².

The Nyquist plot (FIG. 9D) was recorded to examine the charge transferresistance for ZnO, GaON and GaON/ZnO at the interface. All materialsshowed the characteristic semicircular resistance curve, indicating thatthey all are feasible for charge transfer [Y.-H. Lai, C.-Y. Lin, H.-W.Chen, J.-G. Chen, C.-W. Kung, R. Vittal, K.-C. Ho, Fabrication of a ZnOfilm with a mosaic structure for a high efficient dye-sensitized solarcell, J. Mater. Chem. 20 (2010) 9379; and M. Zirak, M. Zhao, O.Moradlou, M. Samadi, N. Sarikhani, Q. Wang, H.-L. Zhang, A.Z. Moshfegh,Controlled engineering of WS₂ nanosheets-CdS nanoparticle heterojunctionwith enhanced photoelectrochemical activity, Sol. Energy Mater. Sol.Cells 141 (2015) 260-269, each incorporated herein by reference in theirentirety]. The radii of the semicircular curves, which represent theelectron transfer at the boundary of the electrode and deposited layers,decreased in the order of ZnO NRs > GaON nanosheets > GaON/ZnOnanoarchitecture. Based on the EIS Nyquist plots, it is clear that afaster interfacial charge transfer is possible in case of GaON/ZnO, andthus it showed much enhanced photoactive behavior than pristine ZnOnanorods and GaON nanosheets as revealed by LSV and chronoamperometry.Table 1 provided a comparison between synthetic methods and photocurrentdensities obtained from PEC water splitting using various Gallium (Ga)based photocatalysts. It can be concluded that pristine gallium oxide(Ga₂O_(s)), gallium nitride (GaN) and gallium oxynitride (GaON) showedrelatively lower current densities as compared to their nanocomposites.Similarly, BiVO₄/GaO_(x)N_(1-x), which was synthesized by a sol-gel spincoating technique, also showed a lower photocurrent density of 0.37mA/cm² at 1.23 V. The ZnO based GaN/ZnO showed a significantphotocurrent density of 0.9-1.2 mA/cm². Nevertheless, the nitridationwas performed by thermal treatment of GaN/ZnO NPs under a harsh NH₃environment, over a relatively expensive TiO₂ substrate.

TABLE 1 Comparative photoelectrochemical water splitting performance ofGallium based photocatalysts in terms of photocurrent density MaterialsSynthesis method Electrolytes Applied voltage (V) Photocurrent density(I) Ref 1. β—Ga₂O₃ NPs RF Sputtering 1 M NaCl 0.2 V vs Ag/AgCl 0.002mA/cm² I 2. Flat porous GaN Thermal Ammonia Treatment 0.5 M Na₂SO₄ 1.23V vs Ag/AgCl 0.20 mA/cm² II 3. GaON nanosheets Hydrothermal Method 0.5 MNa₂SO₄ 0.60 V vs SCE 0.28 mA/cm² III 4. BiVO₄/GaO_(x)N_(l-x) Sol-GelSpin Coating 0.5 M K₃PO₄ 1.23 V vs Ag/AgCl 0.37 mA/cm² IV 5. GaN/ZnO NPsThermal Ammonia Treatment 0.5 M K₃PO₄ 1.23 V vs SCE 0.9-1.2 mA/cm² V 6.GaON/ZnO NTAs Chemical Mixing 0.5 M Na₂SO₄ 1.23 V vs SCE 1.2 mA/cm² Thiswork [Ref. I: S.-J. Chang, Y.-L. Wu, W.-Y. Weng, Y.-H. Lin, W.-K. Hsieh,J.-K. Sheu, C.-L. Hsu, Ga2O3 films for photoelectrochemical hydrogengeneration, J. Electrochem. Soc. 161 (2014) H508-H511; Ref. II: H.J.Kim, J. Park, B.U. Ye, C.J. Yoo, J.-L. Lee, S.-W. Ryu, H. Lee, K.J.Choi, J.M. Baik, Parallel aligned mesopore arrays in pyramidal-shapedgallium nitride and their photocatalytic applications, ACS Appl. Mater.Interfaces 8 (2016) 18201-18207; Ref. III: N. Iqbal, I. Khan, Z.H.Yamani, A. Qurashi, Sonochemical assisted solvothermal synthesis ofgallium oxynitride nanosheets and their solar-drivenphotoelectrochemical water-splitting applications, Sci. Rep. 6 (2016)32319; Ref. IV: B.K. Kang, G.S. Han, J.H. Baek, D.G. Lee, Y.H. Song, S.Bin Kwon, I.S. Cho, H.S. Jung, D.H. Yoon, Nanodome structured BiVO₄/GaO_(x)N_(1-x) photoanode for solar water oxidation, Adv. Mater.Interfaces 4 (2017) 1700323; and Ref. V: Z. Wang, J. Han, Z. Li, M. Li,H. Wang, X. Zong, C. Li, Moisture-assisted preparation of compactGaN:ZnO photoanode toward efficient photoelectrochemical wateroxidation, Adv. Energy Mater. 6 (2016) 1600864, each incorporated hereinby reference in their entirety.]

The obtained PEC water splitting results can be helpful to proposepossible water splitting and charge transfer mechanism in GaON/ZnOnanoarchitecture (FIG. 10 ). An earlier study indicated the valence band(VB) of ZnO lie well below the ideal OER conduction level of watersplitting and therefore showed less significant water splitting [A.G.Tamirat, J. Rick, A.A. Dubale, W.-N. Su, B.-J. Hwang, Using hematite forphotoelectrochemical water splitting: a review of current progress andchallenges, Nanoscale Horiz. 1 (2016) 243-267, incorporated herein byreference in its entirety]. On the other hand, GaON is a goodphotoactive material for HER [N. Iqbal, I. Khan, Z.H. Yamani, A.Qurashi, Sonochemical assisted solvothermal synthesis of galliumoxynitride nanosheets and their solar-driven photoelectrochemicalwater-splitting applications, Sci. Rep. 6 (2016) 32319, incorporatedherein by reference in its entirety]. It was assumed that due toGaON/ZnO formation, these energy bands reorient to a certain extent,which lead to an increase in the water splitting efficiency. During thiscontact, the band structure of GaON/ZnO reconfigures, as the bandbending takes place from GaON to ZnO and drifting of electron startsfrom the former towards later. In addition to band bending, the valenceband of ZnO shifts upward, due to intermixing of their orbitals withGaON nanosheets in the GaON/ZnO nanoarchitectures. The shifting ofenergy levels continues until the Fermi levels achieve a newequilibrium. Upon photon irradiation from the solar simulator, photo-excitons are generated in the valance and conduction band. The negativeelectrons (e⁻) leave behind positive holes (h⁺) in the valence band andjump to the conduction band. These electrons drift from GaON bendingsites to ZnO surface from where it transfers to counter electrodethrough an external circuit. These electrons are readily available forreduction reaction to generate hydrogen from water. On the other hand,the holes (h⁺) in the VB bands of GaON migrate to the VB of ZnO in asimilar fashion and consume the water molecules for OER. As indicated bythe Nyquist plot in FIG. 9D, these photogenerated exciton pairs (h⁺ ande⁻) are effectively separated from each other and therefore, contributeto the PEC water splitting reaction. The PL spectrum also supports thisphenomenon, as it shows dramatic quenching, which can be correlated withthe enhancement of lifetime of photogenerated charges. Thus, GaONextensively enhances the overall water splitting performance of ZnO byshifting the bands to desirable values, providing photogeneratedexcitons, and offering certain stability due to their intercalation andstrong anti-photocorrosive nature.

Example 9 Theoretical Calculations

For theoretical water splitting calculations, spinel structure (Imm2,no. 44) of gallium oxynitride (Ga₃O₃N) with lattice parameters a, b =5.8534 Å and c = 8.2780 ̊Å was considered [T.D. Boyko, C.E. Zvoriste, I.Kinski, R. Riedel, S. Hering, H. Huppertz, A. Moewes, Anion ordering inspinel-type gallium oxonitride, Phys. Rev. B 84 (2011) 85203,incorporated herein by reference in its entirety]. Only atomic positionswere relaxed during the structural optimization. After optimization, aslab geometry with O-rich (111) surface orientation was created, whichwas found to be the lowest energy surface in the experiments. This slabgeometry was further optimized by fixing some of the atoms at the bottomlayer (see FIG. 1B).

To understand water adsorption on the considered systems, a single watermolecule was introduced in the vacuum region at a distance of 5 Å abovethe surface. Structural optimizations were conducted for 8 differentinitial locations of the water molecule using LBFGS optimizer method[D.C. Liu, J. Nocedal, On the limited memory BFGS method for large scaleoptimization, Math. Program 45 (1989) 503-528, incorporated herein byreference in its entirety]. Some atoms at the bottom of the slab werekept fixed during the simulations. The adsorption occurred through theformation of Ga—O or Zn—O bonds.

Black columns in FIG. 11A show the adsorption energies E_(ads) of thewater molecule on the (111) surface of GaON. The water molecule ischemisorbed with adsorption energy less than 2 eV with the formation ofthe Ga—O bond (see FIG. 11B). Dissociation of the water molecule duringthe structural optimization was also achieved (FIGS. 11C and 11D), whichgives the adsorption energies of more than 4 eV. One hydrogen atom ofthe water molecule was transferred to neighboring O or N atoms.Interestingly, the water molecule dissociateed into one hydroxyl and onehydrogen in the GaON+Zn₃O₃ nanoarchitecture system whenever the watermolecule was adsorbed on the Zn₃O₃ cluster (see FIGS. 11E and 11F).Consequently, the adsorption energies become more than 4 eV (seeGaON+Zn₃O₃ columns in FIG. 11A). Molecular adsorption was found in thissystem only when the water molecule interacted with the Ga atoms (FIG.11G). Thus, Zn₃O₃ cluster plays an essential role as a catalytic centerfor the water splitting process.

Example 10

The GaON/ZnO photoelectrode has demonstrated an improved PEC watersplitting performance. Unlike high-temperature ammonia treatment, whichrequires complex safety measures, the incorporation of nitrogen for thefabrication of GaON was carried out at a mild temperature [K. Maeda, T.Takata, M. Hara, N. Saito, Y. Inoue, H. Kobayashi, K. Domen, GaN:ZnOsolid solution as a photocatalyst for visible-light-driven overall watersplitting, J. Am. Chem. Soc. 127 (2005) 8286-8287, incorporated hereinby reference in its entirety]. Composite materials with heterogeneoussystems provide additional reaction sites, thus acceleratingphoto-catalytic reactions [X. Zong, H. Yan, G. Wu, G. Ma, F. Wen, L.Wang, C. Li, Enhancement of photocatalytic H₂ evolution on CdS byloading MoS₂ as cocatalyst under visible light irradiation, J. Am. Chem.Soc. 130 (2008) 7176-7177, incorporated herein by reference in itsentirety]. Such composite materials usually have complex structures,which are difficult to be identified experimentally. First-principlesdensity functional theory (DFT) calculations are recognized as anexpedient tool to study the structural, physical and chemical propertiesof nanoarchitecture systems and obtain a fundamental understanding ofthe mechanism of photocatalytic reactions [M. Setvin, U. Aschauer, P.Scheiber, Y.-F. Li, W. Hou, M. Schmid, A. Selloni, U. Diebold, Reactionof O2 with subsurface oxygen vacancies on TiO₂ anatase (101), Science341 (2013), incorporated herein by reference in its entirety]. Tocomplement our experimental results on GaON/ZnO architecture, DFTcalculations were also conducted on a model system having Zn₃O₃ clustersloaded on (111) surface of GaON (see FIG. 1 ). It was found that theZn₃O₃ model provides efficient reaction sites and enhances thedissociative adsorption energy of water molecules. The DFT resultsconfirm the experimental performance of GaON/ZnO photoelectrode in thePEC water splitting process.

GaON/ZnO nanoarchitecture photoelectrode was successfully developed bythe hydrothermal method over FTO in a single-step process. Themorphological study through FE-SEM indicated GaON nanosheetsinterpenetrated in ZnO nanorods framework. EDX, mapping, and XPSconfirmed elemental details and presence of nitrogen in the overallnanoarchitecture. XRD and Raman spectra showed slight right shift due toreplacement of Zinc by denser Gallium atoms in the crystal due to theformation of GaON/ZnO nanoarchitecture. The optical study via DRS and PLconfirmed the decrease in the bandgap from ZnO (3.26 eV) to GaON/ZnOnanoarchitecture (2.58 eV), as well as significant quenching of GaON/ZnOnanoarchitecture, which might be attributed to visible light absorptionand decrease in the photogenerated exciton recombination rate,respectively. The photoelectrochemical results indicated that thecurrent density was increased by up to 2.4 folds in the GaON/ZnOnanoarchitecture at 1.0 V bias and a maximum current density of 1.2mA/cm² was achieved at 1.2 V. The Nyquist plot showed the decrease inthe resistance of GaON/ZnO, which can be correlated with enhancement ofcharge transfer through the interface. DFT and MD calculations on wateradsorption on GaON and GaON+Zn₃O₃ nanoarchitecture surfaces indicatedthat the catalytic performance of the system increased significantly dueto the presence of the Zn₃O₃ cluster. Our simulation findings are inagreement with the experimental results. It is important to note thatthis approach can be potentially extended to develop othermulti-component cost effective photoactive materials for exceptionallyefficient and stable solar-driven water splitting applications.

1. (canceled)
 2. The photoelectrochemical cell for water splitting ofclaim 10, wherein the zinc oxide nanoparticles are in the form ofnanorods.
 3. The photoelectrochemical cell for water splitting of claim10, wherein the gallium oxynitride nanoparticles are in the form ofnanosheets.
 4. The photoelectrochemical cell for water splitting ofclaim 2, wherein the nanorods have an average diameter of 20-100 nm andan average length of 500-2,000 nm.
 5. The photoelectrochemical cell forwater splitting of claim 3, wherein the nanosheets have an averagethickness of 5-50 nm and an average length of 200-1,500 nm.
 6. Thephotoelectrochemical cell for water splitting of claim 10, wherein aweight ratio of the zinc oxide nanoparticles to the gallium oxynitridenanoparticles is in a range of 4:1 to 100:1.
 7. The photoelectrochemicalcell for water splitting of claim 10, wherein the metal oxide conductingsubstrate is fluorine doped tin oxide film.
 8. The photoelectrochemicalcell for water splitting of claim 10, wherein the GaON/ZnOphotoelectrode has an ultraviolet visible absorption with an absorptionedge of 410-520 nm.
 9. The photoelectrochemical cell for water splittingof claim 10, wherein the GaON/ZnO photoelectrode has a band gap energyof 2.2-2.7 eV.
 10. A photoelectrochemical cell for water splitting,comprising: a GaON/ZnO photoelectrode, wherein the GaON/ZnOphotoelectrode comprises: a metal oxide conducting substrate selectedfrom the group consisting of a fluorine doped tin oxide film and anindium tin oxide film; and a nanoarchitectured photocatalytic materialdeposited onto a surface of the metal oxide conducting substrate;wherein the nanoarchitectured photocatalytic material comprises: zincoxide nanoparticles; and gallium oxynitride nanoparticles interspersedin the zinc oxide nanoparticles; wherein the zinc oxide nanoparticlesform a framework; wherein the gallium oxynitride nanoparticles areinterpenetrated in the framework; and the gallium oxynitridenanoparticles are uniformly distributed in the framework; a counterelectrode; and an electrolyte solution comprising water and an inorganicsalt in contact with both the GaON/ZnO photoelectrode and the counterelectrode.
 11. The photoelectrochemical cell for water splitting ofclaim 10, wherein the electrolyte solution has an inorganic saltconcentration of 0.05-1 M.
 12. The photoelectrochemical cell for watersplitting of claim 10, wherein the GaON/ZnO photoelectrode has aphoto-current density in a range from 0.01-1.5 mA/cm² when thephotoelectrochemical cell is subjected to a potential of 0.1 to 1.5 Vunder visible light irradiation.
 13. The photoelectrochemical cell forwater splitting of claim 10, further comprising: a reference electrode.14-20. (canceled)